Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. In those communication applications at below 6 GHz frequencies, the antennas become increasingly larger. At very low frequencies, for example, used by submarines or other low frequency communication systems, the antennas become very large, which can be unacceptable. It becomes increasingly important in these communication applications to reduce not only antenna size, but also to design and manufacture a reduced size antenna having a relatively high gain for a relatively small area.
In present day communications devices, many different types of patch antennas, loaded whips, copper windings (helix and spiral) and dipoles are used in a variety of different ways. These antennas, however, are sometimes large and impractical for a specific application.
Printed circuit or microstrip patch antennas can be manufactured at low costs and have been developed as antennas for the mobile communication field. The flat antenna or thin antenna is configured, for example, by disposing a patch conductor cut to a predetermined size over a grounded conductive plate through a dielectric material. This structure allows an antenna with high efficiency in a several GHz frequency band to be fabricated in a relatively simple structure. Such an antenna can be easily mounted to appliances, such as a printed circuit board (PCB).
Loop antennas are another form of small antenna. They can be formed of copper rod or tubing bent into a circle. Low operating frequencies can be accomplished by placing a loading capacitor at a discontinuity in the loop ring. At lower and lower frequencies however, the radiation resistance of the loop becomes less than the conductor loss resistance, and low radiation efficiency and gain results. Metals exhibit finite conductivities at room temperature, and conductor loss resistance is a fundamental limitation to the gain and efficiency of small antennas.
However, none of these approaches focuses on reducing the size of the antenna, by providing increasing efficiency and gain in a smaller area. Furthermore, antennas with solid metal conductors suffer from RF skin effect which is a tendency for alternating current (AC) to flow mostly near the outer surface of a solid electrical conductor as the frequency increases. RF skin effect greatly reduces the useful amount of conductor cross section, e.g. in a loading coil wire or loop antenna ring. RF skin effect is a limitation to the gain and efficiency of small antennas.